Frequency shift keying



Feb. 8, 1949.

Filed Feb. 11, 1944 G. L. USSELMAN FREQUENCY SHIF'J. KEYING 3 Sheets-Sheet l ANTENNA AM PLIFI ERS AND/OR FREQUENCY MULTI PLI ER ion No.1

CRYSTAL OSCILLATOR s- SIGNAL CONTROLLED 50 w SOURCE' 5 OSCILLATOR CRYSTAL OSCILLATOR CRYSTAL OSCILLATOR CRYSTAL OSCILLATOR v SIGNAL souaca AMPLIFIER INVEN TOR. GEORGE L. USSELMAN.

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Feb. 8, 1949. G. L USSELMAN FREQUENCY SHIFT KEYING Filed Feb. 11, 1944 3 Sheets-Sheet 5 ANTENNA Y AM PLI Fl ER 1 do CRYSTAL OSCI LLATOR CRYSTAL 05C I LLATOIZ GEORGE LZ.USSELMAN.

ATTORNEY Patented Feb. 8, 1949 2,461,456 FREQUENCY smr'r KEYING George L. Usselman, Port Jefferson, N. signor to Radio Corporation of America, poration of Delaware a cor- Application February 11, 1944, Serial No. 521,907

Claims. (Cl. 250-8) This invention .concerns frequency modulation and in particular an improved method and means of communication by frequency shift or spaced wave keying. By this method of communication carrier wave energy keyed from aflrst to a second frequency and vice verse. is produced. The first frequency may represent marking condition while the second frequency may represent spacing condition. The carrier wave energy so keyed may b considered as originating from a single source the frequency of which is alternately keyed or may be considered as separate carriers alternately translated, the alternation being in accordance with signals such as, for example, telegraphy, facsimile, code or television. Various embodiments of my invention are disclosed herein and in my U. S. application Serial #646,069, filed February 7, 1947, now Patent No. 2,457,288, granted December 28, 1948, which is a division of this application.

At the receiver both frequencies, that is, the mark and space or first and second frequencies are used in reproducing the signal thus giving the benefits of positive action on mark and space, the benefit of frequency diversity, etc.

The use of two frequency sources for frequency shift keying as described above, has been disclosed in the art, as has been the use of the mark and space frequencies at the receiver. However, method and means known heretofore for accomplishing the keying of the two carrier waves resulted in amplitude keying thereof with all'its attendant disadvantages. In the prior systems, even if amplitude keying were not present or did not accompany the frequency shift keying the change from the first to the second frequencies is so; abrupt that there results a wide dispersion of the side frequencies. This extends the channel, causes overlapping of-side bands and produces keying clicks, etc., in the circuits.

An object of the present invention is improved frequency shift keying. An additional object of present frequency shift system retains the advantage of accurate fixed frequencies for the mark and space characters of theprior art, but none of the disadvantages.

In describing my invention in detail, reference I will be made to the attached drawings wherein Fig. 1 is a block diagram illustrating schematically the general arrangement of a spaced wave keying'system arranged in accordance with myinvention.

Fig; 2 illustrates diagrammatically the essential elements of'a practical spaced wave'keying system such as illustrated schematically in Fig. 1. In Fig. 2, two crystal oscillators are arranged to be continuously Operable at .the substantially fixed first and second frequencies and to in turn alternately operate through an electronic and mechanical switching arrangement to synchronize a'third oscillator producing the controlled space and mark frequencies. later is of the condenser resistance type.

Fig. 3 is a wiring diagram of the essential features of a third embodiment of my improved sys-- tem. In Fig. 3 a modified controlled or synchronized oscillator is used and the switchingsystem for switching the control of the said oscillator alternately to the two fixed frequency oscillators is of an improved type operating entirely electronically.

Fig. 4 is a modification Fig. 3; and v Fig. 5 illustratesa modified control circuitjto be used in the embodiments illustrated in Figs.- 3and-4. 3

Referring now to Fig. 1, two crystal oscillators l0 and are provided, onefor the markirequency and the other for the space, frequency. The energies from these two crystal oscillators are of the arrangement of the invention is improved frequency shift keying I without amplitude modulation of the keyed wave.

so that'in effect the keying from the first frequency to the second frequency is smoothly 'car-' ried out, thereby reducing the side band frequency spectrum, reducing keying shocks, etc.

In attaining'the above objects I use two con stant frequency sources, but a method and means is included for the frequency shift keying that holds the amplitude keying modulation to a minimum and also prevents sudden frequency changes.

My method and means thereby prevents unnecessary widening of the frequency band. Thus the in 4i). The switch in 40 may, as will be seenherev inafter,- provide some amplifying effect.

The switch in 40 is controlled by'the signal from source 30 and as. is known, this signal comprises electrical energy representing mark and electrical energy representing space, so utilizedthat first one crystal oscillator outputthen the other is switched onor through to deliver crystal controlled excitation to a controlled or synchronized oscillator in 50. The oscillator in may, when uncontrolled, operate at a frequency intermediate mark and space, but is of a nature such that it is smoothly pulled into synchronism by whichever source In or 20 is switched through at .40 under the control ofs'ignals at 30. The mark and. space frequency energy is then amplified and/or 'fre-' The third oscilquency multiplied in 33 and utilized say. for example, radiated.

The oscillator 50 is of a type that while it oscillates continuously it is easily controlled and synchronized by either of the crystal controlled oscillations switched on by unit 40. Oscillator 50 is adjusted so that during the intervals of switching it continues to oscillate on a mean frequency with substantially no change in amplitude. The output of controlled oscillator unit 53 is next amplified and/or multiplied in frequency in unit 63 and then radiated by the antenna A. The oscillator in 50 is arranged so that it maintains a substantially constant frequency output by self excitation but readily changes its frequency to synchronize with that of the crystal excitation switched on .by the signal from 30 through unit I.

It can be seen that the oscillator 50 provides a buffer or in effect a fly-wheel between the two crystal oscillators Ill and 23, and the transmitter amplifiers and/or frequency multipliers in 33. The action is two-fold. The oscillations continue at the mean frequency and at substantially constant amplitude during the time when neither crystal is in control. Thus, the receiver has a carrier to operate on continuously. The second action is that several cycles of oscillation in unit 53 occur during the time of frequency change from mark to space and vice versa. Thus the keying is smooth since the time frequency change is not too sudden and side frequencies are not spread so far. Note that actually in my system three frequencies are involved, these frequencies being mark, space and no signal.

In Fig. 2, I have shown a more detailed embodiment of the invention. Here the tube VI operates in a well known condenser-resistance oscillator circuit and the condenser-resistance oscillator circuit shown may be included in the unit 50 of Fig. l. The oscillator tube VI has its anode and controlling electrode coupled by a condenser-resistance network including between the plate and control grid series condensers CI, C2, C3 and C4, and between terminals of said condensers and the cathode, resistors RI, R2, R3, R4 and R5 with a coupling and direct current 4 a The crystal controlled oscillators in units l3 and 23 are coupled by coupling condensers l3 and ii to the control grids oi the tubes V2 and V3 while low radio frequency potential points on.

both of the crystal oscillators in units I3 and 23 are connected to ground and to the cathodes of tubes V2 and V3 by a resistance R3 shunted by a coupling and bypassing condenser 0. Direct current grid circuits for V2 and V3 are completed through the resistances RIO and RI I.

The output signals of the signal source at which may be uni-directional current or potentials of opposite or reversible polarity representing mark and space respectively, are supplied to the winding of a relay RE having an armature 2i cooperating with two contacts 22 and 23 to connect the positive terminal of the direct current source to the screen grid of tube V2 or to the screen grid of tube V3 by way of resistance RI2 and one or the other of contacts 22 or 23, depending upon whether mark or space signal is being supplied from source A.

The frequencies of the oscillations developed in III and 2c are usually made to have the deblocking condenser 05 between RI and R2 to isolate the cathode of the tube VI with respect to the direct current potential supplied to the anode of VI through RI. Bypassing condenser 0 serves a somewhat similar purpose with respect to the direct current supplied to the screen grid through potentiometer resistance R. Oscillations are generated by virtue of the fact that the anode of VI is coupled to the control grid by a network such that the voltages on the control grid and anode are opposed. Moreover, the oscillator, while stabilized as to a particular frequency, is readily swung under control of the synchronizing oscillations about this particular frequency, to which it is tuned, by the signals operating on the switch.

Tubes V2 and V3 work in the amplifier and switching circuit which may be included in the switching unit 40 in Fig. 1. The tubes V2 and V3 have their anodes tied together and coupled .to the direct current source through resistance B, the positive terminal of-which is grounded.

The screen grids of tubes V2 and V3 are also connected by radio frequency bypassing condensers C to the cathodes.

sired prescribed frequency difference of about or more cycles, depending upon the needs of the particular system at hand. The relay RE is operated from the signal source A. The relay contacts 2i, 22 and 23 alternately change the screen grid bias potentials of the switching and amplifying tubes V2 and V3 from a negative to a positive value. When contact 22 or 23 is closed the screen grid of tube V2 or V3, depending upon which contacts is closed, is connected to the positive terminal of the source. This alternately switches excitation from crystal units III and 23 onto the oscillator stage tube VI because the oscillations are amplified or repeated through one of the tubes V2 and V3 and supplied to the screen rid circuit of tube VI. In the mid position of the relay RE the contacts 22 and 23 are open,

and negative bias is applied to both switching tubes V2 and V3 from source B through the resisters R1 and R3. In this condition no excitation from either crystal oscillator is allowed to pass through the amplifying and keying stage. During this time oscillator VI is operating on its frequency determining circuit, that is, the condenser resistance network which is set for the mgan frequency and this frequency is transmitte Now assume that the signal from A closes the relay contacts 2I and 22. Positive bias is applied to the screen grid of amplifying and switching tube V2, which repeats and possibly amplifies the excitation from crystal oscillator I 3, impressing it on the screen grid of oscillator tube VI through potentiometer R3. Since the oscillator VI is as described above, somewhat flexible in operation, it has its frequency "pulled so that it synchronizes with the frequency of crystal oscillator I 0. This new frequency is now transmitted and may represent space or mark. when the signal from source A reverses the relay RE changes and closes contacts 2| and 23. During the changeover period there is a short interval of time when negative bias is applied to both tubes V2 and- V3, and the excitation from both crystal oscillators I3 and 2c is cut oil. Then the oscillator VI swings smoothly back towards its own frequency ofoperation, that is, mid-frequency. When the relay contacts 2I and 23 are closed positive bias is applied to the switching tube V3 which repeats and possibly amplifies the excitaenemas 20 before the oscillations developed in tube VI are locked in synchronism. By adjusting the P tentiometer R6 the amount 01' control excitation potential from the crystal oscillators in ID and to the controlled oscillator VI may be set'to give the desired pull-in or synchronizing time. The effect is a rounding off and slope in the signal characters. Reducing th excitation voltage will increase the rounding and slope of the signal characters. Too much reduction will cause loss of signal or erratic operation. 'If a large amount of excitation is impressed on oscillator tube VI from the keying tubes, the signal characters will be more square shaped .at the v corners and more side frequencies will be generated. During the shifts in frequency theoutput of oscillator VI remains substantially constant in amplitude.

Although. as pointe. out above, the shift between mark and space frequencies is smoothly carried out, it must be understood that the action is positive under all circumstances provided, of course, sufllcient control is appl ed at R6 to synchronize the oscillator VI wih one or the other of the sources in I0 and 20. The actionis positive because when tube V3 is keyed on, that is, has its screen grid connected to the positive source of direct current by contacts 2| and 23, tube V2 is shut oil due to the fact that its screen grid is connected by resistance R1 to the negative terminal of source B.

On the other hand, when the tube V2 has its screen grid supplied with positive potential by contacts 2| and 22, tube V2 is blocked due .to the fact that its screen grid is connected by resistance R8 to the negative terminal of source B.

The arrangement of Fig. 3 is quite similar to the arrangement of Fig. 2. The main diflerence between the arrangements of Figs, 2 and 3 results from the fact that a different type of synchronized or entrained oscillator including tube VI is used with additional control means therefor.

In Fig. 3, VI is an oscillator of the Hartley type having its anode coupled to its control grid by a tuned LC circuit including condenser CT and inductance Ll with a point on th inductance LI connected to'the cathode by coupling condenser O. Oscillations are generated in the tube VI and circuit CT, LI by virtue of the fact that enough electrons are drawn past the control grid at all times to support oscillations. The condenser N is a feedback or regeneration control condenser .and is a resistance by .means of which operating bias is supplied to the control rid of tube VI.

The potentiometer 21' has a tap thereon and provides a means for applying the synchronizing or controlling voltage to the generator of tube VI. Any excitation applied to the control grid or any tube element of the oscillator tube VI will have a tendency to control the frequency of oscillations generated in tube VI, thereby-synchronizing the same with the controlling potentials, provided the natural oscillating frequency of the tube VI is not too far different from the excitation or control frequency. A controlled oscillater of this general type has been shown and described in detail in Fig. 3 of my U. S, Appln. Serial #487,095, filed May 15. 1943, now Patent No. 2,425,165, granted August 5, 1947- In my improved system a tap on the potentiometer resistor 21' in the cathode return circuit of tube VI is coupled to the anodes of the tubes V2 and V3, by way'of direct current blocking and alternating current coupling condenser 0 Control in Fig- 3 is accomplished in an improved manner by applying the output of one or the other of the amplifying and keying tubes V2, V3 to a potentiometer 21' in the cathode to ground circuit of the oscillator so that this oscillator VI is synchronized through control of the potential on the cathode thereof. The grid direct current circuit is completed by gridleak resistance 25'. The anodesof tubes V2 and V3 are coupled to IP'through coupling and direct current blocking condenser 05 over resistance 25.

'The switching circuit now takes the form of a tripping type keying circuit thereby also eliminating the .use of the mechanical relay RE. In this tripping circuit tubes V4 and V5 are arranged and operate substantially as disclosedin U. S. Patent #2326314, dated August 10, 1943. The anodes of the tubes are cross-connected to the control grids by resistances H and 42 in such a way that the potential drops in anode resistances 42 and 44 are applied respectively to the grids of thetubes V5 and V4. When one of these tubes draws current the grid of the other thereof is -made more negative by applying the potential drop at .the anode of the said one tube to the grid of the other tube.

The anodes are also coupled together by resistances 4 3, 44, and and 46. A point intermediate the resistances 43 and 44 is connected to the positive terminal of the direct current source while a point intermediate the terminals of resistances 45 and 46 is connected to a point on a potentiometer 49 in shunt to the source 282. The source B2 has its positive terminal grounded like source 13 of Fig. 2, and this source B2 is shunted by a bypass condenser O. The resistances 45 and 46 which are potentiometers have points thereon coupled to the screen grids in tubes V2 and V3, and are means for supplying alternatingly positive and negative bias to the screen grids of keying tubes V2 and V3 for keying the same on and oil as described hereinbefore. The outer ends of resistances 45 and 43 are connected to an alternatingly varying positive voltage, that is, to the anodes of tubes V4 and V5. The anodes are made alternatingly more positive and less positive as the bias on the grids of the. tubes V4 and V5 is keyed. The central ends .of these resistors are connected to a negative source of suppl by resistance 49.

The tripping tubes V4 and V5 have their control grids coupled respectively by resistances 58 end. and 59 and 83 to ground and by resistance 65.110 the cathodes of the tubes. Re,-

sistancei! is shunted by a condenser 61 of low impedance to potentials of the keying frequency.

BI and 63 are potentiometer resistances with a point on resistance 8| connected by switch 5 either to the negative terminal of a sourcem. or to ground. A point on potentiometer resistance 63 is connected to one terminal ofthe keying source, the other terminal of which is connected to ground. The potentiometer-s BI and 62 couple in the control or keying potentials. The

source B2 is sometimes necessary to apply a fixed negative bias to the grid of tube V4 in order to get positive tripping action. More speciflcally, the setup is such that the tube V4 is biased to cutofl in the absence of marking and spacing signals either by the drop in resistances 58 and BI or if necessary by negative potential from B3 with S on SI instead of'on S2. The current then is switched through tube V5 and the resistances 45 and 46, the tap on source 49, etc., is such that the screen grid of tube V3 is negative to block this tube at the same time the anode of tube V4 is at a positive potential (main D. C. source) such that the screen grid of tube V2 is positive, and oscillations from source III are amplified through V2 to take control of the generator tube VI. This frequency of operation of source III and VI as now "controlled couldbe 1 tube V5 and from the end of resistance 44 through the tap on resistance 46 to the screen grid of tube V3 and. the control of tube Vi shifts from source It'to source which may then be said to operate go at the marking frequency, and the generator VI now is entrained by source 20. The keying potentialsapplied at H and" may be alternating currentor direct current pulses. Alternating potentials' may be applied to the input II .of the tripping circuit for keying.

When alternating current and voltage is applied across terminals II' and I3 of Flg. 3 it is usually done through a transformer. During one part of the cycle the grid of tube V5 will be biased positive and during the other. part of trip to high and low positive voltages. If the movable points on potentiometers 4i and 48 are correctly adjusted the screen grids oiswitching tubes V2 and V3 will have impressed on them alternately positive and negative bias voltages in accordance with the signal potential variations. The adjustments of potentiometers 45, 4B, and 49 control these biasing potentials-and adjustment can be-made to make the-switching action as positive as desired;

Advantage of this modification is that there are no mechanical relays as in Fig 2. The tripping circuit squares up the marking; pulses and the same are amplitude limited by .the tripping circuit. The tripping circuit also operates =-so that there is a minimum of transition time between mark and space frequencies. ,In, other words, the time of uncontrolled oscillation in the circuit of VI is a minimum.

a The condensers 0 connecting the keyed screen grid electrodes of tubes V2 and V3 to the cathodes of the respective tubes in Figs. 2, 3, and 4, are bypass condensers for voltages of radio frequency such as generated in sources It and 20,

a but these condensers are not to be so large as to. distort audio frequency-characteristics, as otherwise the keying impulses of higher frequency might be greatly reducedin amplitude.

However, these condensers may be made as large as-.1 MB or 3M1 for the purpose of rounding oif the signal characters if very high keying speeds are not used. In the prior figures and in Fig. 4, to be described hereinafter, the letter 0 is applied to condensers such as between R6 and 5 the cathode in Fig. 2, which serve primarily as 4 quency.

the cycle the grid of tube V5 will be biased by the grid of tube V4 is more negative than the grid of tube V5 when no signal or when positive signal is applied to V5. when a suiiiciently large negative signal pulse is applied to terminal I3, the grid of tube V5 is made suiiiciently more negative than the grid of tube V4 to cause the circuit to. trip so that tube V5 is cut of! and tube V4 carries large current. when terminal I3 goes to zero or a positive potential the circuit .will trip back to the original condition of largecurrent in tube V5 and no current in tube .V4. Switch contact Si or S2 isclosedwhen only negative nu lsesot potential are applied to terminal 13. I! alternating bositive'and' negative potential pulses are a plied to terminal 'I3 then switch Sl-S2 may be left open and satisfactory'operation will be obtained. The tripping circuit performs the additional function of squaring up the keying wave form. The arrangement of Fig. 3 performed verywell in operation.

Irrespective of whether the potentials at bypassing condensers but mayalso serve as harm, and depending on the operating fr'e- The embodiment illustrated in Fig. 4 uses a controlled oscillator of the modified Hartley type including tube VI, with switching tubes V2 and V3 coupled thereto and controlled by the tripping circuit including tubes V4 and V5 as illustrated in Fig. 3. The anode of tube VI is coupled to the control grid by a tuned LC circuit ineluding condenser CT and inductance LI with a point on the inductance LI connected to the cathode by a coupling condenser 0. Oscillations are generated in a well known manner in this regenerat ve circuit. The screen grid is connected by coupling condenser I5 to a point on potentiometer resistance R'I ooupledby condenser 05 between the anodes'of tubes V2- and V3 and the cathode of tube Vi. Direct current is su plied for this electrode from the main source by reused. Moreover, the operation'of'the tripping circuit and switching tubesis as described in detail hereinbefore in connection with Fig, 3. It does not appear necessary to repeat this operation in connection with Fig. 4. It will be noted. however. that the embodiment of Fig. 4

operated very well whentested and is now believed to, be a preferred embodiment.

"I'he operation of the embodiments illustrated in Figs. 3 and 4 is made more positive by modI tying the- -dire +.r'current supply circuits for the oscillator including tube VI and the two switchin tubes V2 and V3. as shown in Fig. 5. 4

The audio" frequency choke HI. together with radio frequency choke F and radio frequency bypass condenser 88, is used for the following purpose. Ordinarily without the choke it is possible for the cut off and on of the keying tubes V2 and. V3 to be somewhat ragged or out of step. That is, both might be off at once or on at one time.

Although this seldom occurs, I may wish to provide further assurance that itwill not occur by installing iron corechoke coil Hi as shown in Fig. 5. This choke coil tends to maintain constant current flow so that when one keying tube, say V2, cuts off the difference current is forced to flow in tube V3. Suppose that the .total current in choke coil HI is ll) MA. Then I would have conditions of keying currents like these: V2=10 MA, V3=0 MA; V2='8 MA, V3=2 MA; V2=5 MA, V3=5 MA; V2=0 MA, V3=10 MA, etc. At any tripping speed of the keying tubes these tube currents would be complementary, thus insuring that the keying tubes operate properly at all speeds.- The radio frequency choke F and bypass condenser 88 are provided to prevent the choke HI from absorbing radio frequency power, which it would otherwise do.

Similarly, I may wish to use a choke coil similar to H2 of Fig. 5 in the plate current supply lead to the controlled oscillator circuit and tube Vi. The purpose of choke coil H2 is to maintain constant current flow in oscillator tube VI in order to inhibit any amplitude modulation. For instance, if during the excitation switching operation there is a change of excitation level to tube VI, the same anode current flow is maintained by choke coil H2, which tends to hold constant radio frequency amplitude output.

The tripping circuit of Fig. 3 uses two tubes and has a pushpull output. Its input and output circuits may be either single ended or pushpull as desired.

Pushpull input may be obtained by leaving switch s open and then applying the pushpull signal through switch s to potentiometer tl-and through terminal 13 to potentiometer 63. The pushpull input signal should be balanced positive and negative potentials similar to ordinary alterhating currents and voltages. However, single ended input is satisfactory and will usually answer every purpose. It can be applied to either the upper or the lower side of the input circuit providing the other side is biased.

In the tripping circuit of say Fig. 3, I not only want to adjust the amount of keyed output voltage but I want to adjust the amounts of positive and/or negative voltage supplied by the keyed output. This is done by adjusting the negative voltage supplied by potentiometer l9 and by adjusting the output. connections on potentiometers l5 and 46 in Fig. 3.

If the ground connection of potentiometer 49 in Fig. 3 is moved part way upon the potentiometer, then this tripping circuit may be adjusted to supply modulated positive or negative potential output.

I claim:

1. In a telegraphy system a source of oscillations of controllable frequency, a source of oscillations of a first and substantially fixed frequency, a source of oscillations of a second and substantially fixed frequency, a circuit including an amplifier between each oscillator of fixed 'frequency and said oscillator of controllable frequency and apparatus including a switching device controlled in' accordance with telegraphy and the other amplifier inoperative or vice versa for entraming said source of oscillationsoi controllabie frequency by oscillations from one or the other of said sources of substantially fixedand controlled in accordance with telegraphy signals for alternately rendering said amplifiers operative for'coupiing one or the other of said sources of substantially fixed frequency to said source of controllablefrequency for entraining the same with one or the other of the substantially fixed sources.

3. In a frequency shift system, a source .of oscillations of controllable frequency, a source of oscillations of a first and substantially fixed frequency, a source of oscillations of a second and substantially fixed frequency, two electron coupling tubes each having input electrodes coupled respectively to one of said sources of substantially fixed frequency, and having output electrodes coupled in parallel and to said first source to entrain the frequency of operation thereof, a source of signals comprising energy the character .of which changes between two conditions representing respectively mark and space, and

connections between said source of signals and electrodes of said pair of tubes for controlling oppositely the conductivity of the tubes as thea character of the signal energy changes.

4. In afrequency shift signalling system, a

regenerative tube generator of controllable frequency, two electron coupling tubes each having input electrodes coupled respectively to one of said sources of substantially fixed frequency,

' and having output electrodes coupled in parallel and to said first source to entrain. the same and control the frequency of operation thereof, a source of signals comprising energy keyed between two conditions representing, respectively mark and space, a second pair of electron discharge tubes each having an output electrode coupled respectively to an electrode of one of said coupling tubes to control the operativeness thereof, connections cross-coupling electrodes of said second named pair of tubes so that when one draws current, current through theother is reduced and vice versa, and connections between I said source of signals and electrodes of said signals for rendering one amplifier operative 18 second pair of tubes for controlling oppositely the conductivity of the tubes as the character of the signal energy changes.

5. In a frequency shift signalling system, a source of oscillations of controllable frequency, a source of oscillations of a first and substantially fixed frequency, a' source of oscillations of a second and substantially fixed frequency, two

electron coupling tubes each having input electrodes coupled respectively to one of said sources of substantially fixed frequency, and having output electrodes coupled in parallel and to said first-source to entrain the frequency of operation thereof, a source of signals comprising energy the character of which changes between two conditions representing respectively mark and space, connections between an electrode of each of said tubes and a negative and a positive source of potential, 9. switch in at least one of said connections to each electrode, and a relay cooperating with said switch and controlled by said signals for applying a negative potential to an electrode in one of said tubes and a positive potential to an electrode in the other of said tubes or vice versa as the character of said signals changes.

6. In a frequency shift telegraphy system, an oscillator of controllable frequency operating to generate oscillations of a first frequency, an oscillator operating at a second and substantially fixed frequency, an electron discharge tube coupling the last mentioned oscillator to the first mentioned oscillator, for entraining the first oscillator by the second oscillator, a source of currents representing two signalling conditions, and connections from said source to said tube to control the coupling through the tube from the second oscillator to the first oscillator in accordance with currents from said source.

7. In a telegraphy system of the type wherein signals are transmitted at a first frequency designated mark and a second frequency designated space, an electron discharge device oscillator having electrodes coupled in alternating cur-' rent circuits and to a source of direct current potential all arranged to produce oscillatory energy or carrier wave frequency, an oscillation generator of fixed frequency, a second oscillation generator of fixed frequency different than the frequency of said first oscillation generator of fixed frequency, two electron discharge tubes each having input electrodes and output electrodes, a coupling between one oscillator of fixed frequency and the input electrodes of one tube, a coupling between the other oscillator of fixed frequency and the input electrodes of the 1 other tube, a connection tying the output electrodes of said tubes together and to a point on the alternating current circuits of said device oscillator, a source of direct current potential coupled to the output electrodes of said tubes, a source of potential which varies with respect to a base value in accordance with telegraphy signals, and a relay actuated by said last mentioned potentials for controlling the conductivity of said tubes differentially in accordance with said last mentional potentials for entraining said first mentioned oscillator by energy developed by one or the other of said two fixed frequency oscillators depending on the value of said last mentioned potentials relative to said base value.

8. In a telegraphy system of the type wherein signals are transmitted at a first frequency designated mark and a second frequency designated space, an electron discharge device oscillator having electrodes coupled in alternating current circuits arranged to produce oscillatory energy of carrier wave frequency, an oscillation generator of fixed frequency, a second oscillator generator of fixed frequency different than the free quency of said first oscillation generator of fixed frequency, two electron discharge coupling tubes having input electrodes and output electrodes, a

, coupling between one oscillator of fixed frequency and the input electrodes of one tube, a coupling between the other oscillator of fixed frequency and the input electrodes of the other tube, a connection tying the output electrodes of said tubes together and to a point on said alternating current circuits, a control electrode in each of said tubes, a source of potential for said control electrodes such as to bias said tubes to substantially cutoff, a source of potential which varies relative to a base value in accordance with telegraphy signals, a relay having a winding coupled to said last named source and having an armature the position of which depends on the.value of said potentials relative tosaid base value, a source of positive direct current connected to said armature, two contacts associated with said armature, a connection between one contact and one control electrode, and a connection between the other contact and the other control electrode.

9. In a telegraphy system of the type wherein signals are transmitted at a first frequency designated mark and a second frequency designated space, an electron discharge device oscillator having electrodes coupled in alternating g5 generator of fixed frequency different than the frequency of said first oscillation generator of fixed frequency, two electron discharge tubes each having input electrodes and output electrodes, a coupling between one oscillator of fixed frequency and the input electrodes of one tube, a coupling between the other oscillator of fixed frequency and the input electrodes of the other tube, a connection tying the output electrodes of said tubes together and to a point on said first mentioned alternating current circuits, a control electrode in each of said tubes, a source of negative direct current potential connected to said control electrodes, a source of potential which varies with respect to a base value in accordance with telegraphy signals, a second pair of electron discharge tubes each having input and output electrodes cross-coupled so that when current flows in one of said last mentioned pair of tubes current is cutoff in the other of the tubes of said last mentioned pair of tubes and vice versa, means for controlling the conductivity of at least one of the tubes in said last mentioned pair in accordance with said telegraphy signals, impedances coupling the output electrodes of the tubes of said second pair of tubes to said control electrodes of the tubes of said first pair of tubes, and a source of positive direct current potential connected to the output electrodes of the tubes of both pairs of tubes.

10. In a telegraphy system of the type wherein signals are transmitted at a first frequency designated mark and a second frequency designated space, an elect on discharge divice oscillator having electrod s coupled in alternating current circuits arranged to produce oscillatory energy of carrier wave frequency, an oscillation generator of fixed frequency, a second oscillation generator of fixed frequency different than the frequency of said first oscillation generator of fixed frequency, two electron discharge coupling tubes each having input electrodes and output electrodes, a coupling between one oscillator of fixed frequency and the input electrodes of one tube, a couplingbetween the other oscillator of the control electrodes of the tubes of said pair of tubes, a source of potential which varies with respect to a base value in accordance with telegraphy signals, a second pair of electron discharge tubes each having input and output electrodes, impedances cross-coupling the input and output electrodes of the tubes of said second pair of tubes so that when current flows in one of said tubes current is cutoff in the other of said tubes and vice versa, a coupling between said variable source of potential and the input electrodes of the tubes of said last named pair of tubes for controlling the conductivity of at least one of said tubes of said last named pair in accordance with said telegraphy signals, and impedances coupling the output electrodes of the tubes of saidsecond pair of tubes to the control electrodes of the tubes of said first pair of tubes and to a source of positive direct current potential.

11. In a frequency shift telegraphy system, a source of oscillations of controllable frequency, a source of oscillations of a first and, substantially fixed frequency, a source of oscillations of a second and substantially fixed frequency, two electron discharge devices having output electrodes and having input electrodes, a coupling between the input electrodes of one device and one of said sources of oscillations of substantially fixed frequency, a coupling between the input electrodes of the other device and the other of said sources of oscillations of substantially fixed frequency, couplings between the output electrodes of said devices and said source of oscillations of controllable frequency to entrain the frequency of operation thereof, a source of signals comprising pulse energy which varies between a first value representing mark and a second value representing space, and connections between said source of signals and electrodes of said pair of devices forv differentially controlling the conductivities of the devices as the pulses vary in magnitude between said two values.

12. In a frequency shift telegraphy system, a regenerative tube generator of controllable frequency, a source of oscillations of a first and substantially fixed frequency, a source of oscillations of a second and substantially fixed frequency, two electron coupling tubes .each having output electrodes and eachhaving input electrodes, a coupling between the input electrodes of one of said coupling tubes and one of said sources of oscillations of substantially fixed frequency, a coupling between the input electrodes of the other coupling tube and the other of said sources of oscillations of substantially fixed frequency,

source of oscillations of a first and substantially fixed frequency, a source of oscillations of a second and substantially fixed frequency, two electron coupling tubes each having output electrodes and each having input electrodes, a coupling between the input electrodes of one coupling tube and one of said sources of oscillations of substantially fixed frequency, a coupling. between the input electrodes of the other of the coupling tubes and the other of said sources of substantially fixed frequency, couplings between the output electrodes of said coupling tubes and said first. source of oscillations to entrain the frequency of operation thereof, a source of signals comprising pulse energy the magnitude of which changes between two values representing respectively mark and space, connections between an electrode of each of said tubes and a negative and a positive source of potential, a switch-in at least one of said connections to each electrode, and a relay cooperating with said switch and controlled by said signals for applying a negative potential to an electrode in one of said tubes and a positive potential to an electrode in the other of said tubes and vice versa as the characteristic of said signals changes.

14. In a frequency shift telegraphy system, a regenerative generator of controllable frequency, said generator comprising an electron discharge device having an anode-like electrode and a control grid electrode regenerativelycoupled for the production of oscillations, biasing circuits for the electrodes of said device including a resistance connecting the cathode thereof to ground,

which changes between two values representing respectively mark and space, and connections between said source of signals and electrodes of said pair of tubes for differentially controlling the conductivity of the tubes as the magnitude of said signals varies between said two values.

15. In a frequency shift telegraphy system, a regenerative generator comprising an electron discharge device having an anode and a control couplings between the output electrodes of said through the other thereof is reduced and vice versa, and connections between said source of v signals and electrodes of at least one pair of said second pair of tubes for keying the same to be conductiveor non-conductive as said signal energy condition changes.

13. In a frequency shift telegraphy system, a

' source of oscillations of controllable frequency, a

that when one thereof draws current, current grid and a cathode regeneratively coupled for the production of oscillations, said device having an additional control electrode coupled by a potentiometer resistance to its cathode, two sources of oscillations of substantially fixed and of different frequencies, two electron coupling tubes each having input electrodes coupled respectively to one of said sources of fixed frequency, and having output electrodes coupled to said regenerative generator to entrain the same at the frequency of one or the other of said sources of substantially fixed frequency, a source of signals comprising pulse chergy which varies in magnitude between two values representing respectively mark and space, a second pair of electron discharge tubcs each having an output electrode coupled respectively to an electrode of one of said coupling tubes to control the operativeness thereof, connections cross-coupling electrodes of said second pair of tubes so that when one draws current, current through the other is reduced and vice versa, and connections between said 18 3 source of signals and the electrodes of at leest one of the tubes of said second pal of tubes for controlling the conductivity thereof to thereby diflerentially control the conductivities of the tubes of the second pair of tubes;

GEORGE L. USSELMAN.

3mm men The following references are of record in the file or this 'patent:

U STA PAH-Ms Number Name 7 Date 785,803 met: Mar. 2a, 1905 r 1,795,393 Herman Mar. 10,1931 1,017,312 Melssn'er min, 1933 Number 10 Number OTHER REFERENCES Radio & Television pp. 288-291, September 1941. 

